The present invention relates to color measurement instruments, and more particularly to color measurement instruments that include modulated LEDs as illumination sources.
A variety of color measurement instruments are well known and widely used in a variety of applications to measure color. Some of these instruments illuminate the target sample sequentially with a plurality of monochrome illuminators, measure the light reflected by the target sample to each of the monochrome illuminators, and determine the color of the target sample based on all of the measurements.
Light emitting diodes (LEDs) have been used as the monochrome illuminators. Original instruments included red, green, and blue LEDs. More recent instruments include more than three LEDs; and some include as many as eight.
In such instruments, the LEDs are typically modulated at predetermined frequencies so that the sensing circuit can discriminate between light reflected from the LEDs, which is of interest, and light reflected from ambient light, which is not of interest. The sensing circuit can ignore the ambient component by looking only at reflected light received at the predetermined frequencies.
Several problems exist in current LED-based instruments. A first problem is that the output of the LEDs varies with the temperature of the LEDs. Specifically, the output changes in terms of intensity, spectral energy distribution, and the spatial distribution of the output. The temperature changes are attributable both to the ambient temperature and the amount of time that the LEDs are illuminated. Unfortunately, this variation in LED output adversely impacts the accuracy of the color measurement.
A second problem is that the ambient light component can saturate the transimpedance amplifiers in the sensing circuit and thereby limit dynamic range, particularly in the first stage. Prior artisans have addressed this problem by placing a shunt element in opposition to the photodiode, integrating the output of the transimpedance amplifier at a frequency less than the ambient light frequencies, and using the integrated signal to control the shunt to act as a current sink for the frequencies of the ambient light. While this is an effective way to cancel the effects of an extremely wide dynamic range of ambient light, it also is inherently noisy and sensitive to loop gain and bandwidth issues. Consequently, measurements include errors of an undesirable magnitude.
A third problem is that the distance between the instrument and the sample is a critical factor that must be precisely controlled. This critical factor is known as positional sensitivity. Because the optics of such instruments are typically tuned to a precise distance, variations in that distance typically detract from the accuracy of measurements. Unfortunately, positional accuracy is not a practical possibility in industrial applications, where positional repeatability varies to some degree because moving components, such as robotics, cannot always be positioned precisely.